The problem can be corrected by upgrading the affected package to
version 4:4.3.8-3ubuntu7.15 (libapache2-mod-php4 for Ubuntu 4.10),
4:4.3.10-10ubuntu4.4 (libapache2-mod-php4 for Ubuntu 5.04), or
5.0.5-2ubuntu1.2 (libapache2-mod-php5 for Ubuntu 5.10). After a
standard system upgrade you need to restart Apache with

sudo apache2ctl restart

to effect the necessary changes.

Details follow:

Stefan Esser discovered that the 'session' module did not sufficiently
verify the validity of the user-supplied session ID. A remote attacker
could exploit this to insert arbitrary HTTP headers into the response
sent by the PHP application, which could lead to HTTP Response
Splitting (forging of arbitrary responses on behalf the PHP
application) and Cross Site Scripting (XSS) (execution of arbitrary
web script code in the client's browser) attacks. (CVE-2006-0207)

PHP applications were also vulnerable to several Cross Site Scripting
(XSS) flaws if the options 'display_errors' and 'html_errors' were
enabled. Please note that enabling 'html_errors' is not recommended
for production systems. (CVE-2006-0208)